1. Field of the Invention
The present invention relates to a method for the manufacture of the gates of field-effect transistors, more particularly dedicated to microwave transistors, for which the gate has to be thin at its base and, at the same time, has an apron or cap that reduces its electrical resistance: these gates are called mushroom gates.
The performance characteristics of a field-effect microwave transistor, as shown schematically in FIG. 1, depend partly on the length L of the gate G, measured along the source S/drain D axis L must be reduced when the frequency is increased. At the same time, they depend on the electrical resistance of the gate which is higher for thinner gates. There is a constant search, in the designing of transistors, for low values of access resistance.
2. Description of the Prior Art
It is for these reasons that mushroom-shaped gate metallizations have been developed. These gates have a base with a small width (in the micronic or sub-micronic range) and an apron or cap that is wider and is given an additional coat of metal to reduce the electrical resistance.
However, the performance characteristics of a microwave transistor in general, and those of a power microwave transistor in particular, are improved by increasing the distance between the gate G and the doped zone n.sup.+ controlled by the drain D: this reduces the gate/drain stray capacitance C.sub.GD and increases the breakdown voltage of the transistor.
A disymmetrical structure such as this is shown in FIG. 1. Depending on whether the transistor has a recess for the gate or is self-aligned, this positional disymmetry of the gate may be obtained by different expedients, among them the deposition of the gate at a slight inclination, or else an additional masking to take the disymmetrical n.sup.+ type implanted zone to a distance from the two source and drain trenches.
However, depending on whether the mushroom gate is disymmetrical or not, its manufacture remains a delicate operation for the following reason, shown schematically in FIG. 2: the most common method consists in the deposition of a thick layer 2 of masking resin on a wafer 1 made of semiconductor materials (comprising the substrate and the different layers constituting a transistor). With an appropriate beam of electrons or X-rays, a first scan 3 is made to define the apron of the gate on a part of the thickness of the resin 2, then a higher energy second scan 4 is made to define the base of the gate throughout the thickness of the resin 2. The different doses may be obtained either by modifying the energy of the beam or by carrying out several passes for the second scan 4. After the irradiated resin has been dissolved, there remains an indentation in which there is deposited the metal of the mushroom gate 6, thickened by electrolytic means. The metal is also deposited at 7 on the upper face of the resin layer 2, and forms fine membranes at 8, adjoining the metallizations 6 and 7.
The drawback of this method arises out of the fact that, when the resin 2 is dissolved during a lift-off operation, the metal film 7 remains soldered to the gate 6 by the membranes 8 because the implantation and the dissolving of a single masking resin cannot give an indentation contour 5 such that there is a sharp break in the contour between the upper face of the resin 2 and the lateral walls of the indentation 5. The rounded portions caused by the lift-off operation means that, at 8, the metal of the film 7 descends into the indentation 5, and is continous with the gate 6.